An idiolect, if there is such a thing, is a language that can be characterised exhaustively in terms of intrinsic properties of some single person at a time, a person whose idiolect it is at that time. The force of ‘intrinsic’ is that the characterisation ought not to turn on features of the person's wider linguistic community. Some think that this notion of an idiolect is unstable, and instead use ‘idiolect’ to describe a person's incomplete or erroneous grasp of their language, where this latter is inherently social. Several important debates have featured discussion of individualistic, personal, or private languages. Some are considered in other entries (see the entries on the language of thought hypothesis, private language, and reference). This entry will concentrate on two influential and broadly idiolectal positions in the philosophy of language and linguistics: Noam Chomsky's preference for I-languages over E-languages (Section 2), and Donald Davidson's rejection of languages conceived as shared conventional structures that make communication possible (Section 3). David Lewis's claim that languages are a convention is a common target for both, and is outlined in an Appendix. The entry begins (Section 1) with some general remarks about the ontology of languages. Contents: 1. Idiolects and Language Individuation; 2. Chomsky on E-languages and I-languages; 2.1 The Origins of Chomsky's Distinction: Competence vs. Performance; 2.2 The Distinction Elaborated; 2.3 Why I-languages? Why Not E-languages?; 2.4 Criticisms of Chomsky's Preference for I-languages Over E-languages; 3. Davidson's Claim That There Are No Such Things As Languages; 3.1 What Davidson Aims to Show; 3.2 The Argument From Malaprops and Related Phenomena; 3.3 Reaction to Davidson's Argument; Bibliography; Other Internet Resources; Related Entries